Magnetic recording media from polymeric compositions containing inorganic fillers

Abstract: A composition comprising an inorganic filler and binder component containing a glassy polymer and a rubbery polymer is provided. The glassy polymer contacts and encapsulates the filler. The compositions are especially useful in the fabrication of flexible magnetic recording media and abrasive articles.

Description: The present invention is concerned with polymer compositions that are reinforced with inorganic filler and used in the fabrication of flexible magnetic recording media. More particularly, the compositions of the present invention provide tough, cohesive, high modulus coatings. According to the present invention, a polymer component containing a glassy resin and a rubbery polymer is employed wherein the glassy resin is in contact with and encapsulates the inorganic filler. Various types of bonding materials are useful in the fabrication of filled synthetic polymer compositions. Certain of these synthetic polymer compositions have been used for fabricating flexible magnetic recording media. Another application of importance, especially from a commercial point of view, involves fabricating abrasive articles. In both of these areas of use, the interactions between the inorganic filler and binder can effect critical characteristics of the final composition, and therefore, its applicability for practical use. This is especially important since in both magnetic recording media applications and abrasive articles, high filler loading is employed which makes it necessary to have strong interactions between the filler particles and binder. In addition, high density recording requires tape coatings to be fabricated with increasingly smaller magnetic particles. However, current binder systems do not adequately address the need for optimized binder-filler interaction and coating mechanical requirements. In fact, current binder systems produce excessive debris and contamination of the recording interface. For instance, it has been particularly difficult to achieve useful reinforcement of conventional binders when filled with stabilized iron metal particles, or of aluminum oxide (alumina) particles, typically included along with iron metal or other non-abrasive particulate media, as head cleaning agents. In addition, due to the lack of significant chemical interaction with the oxide protective layer on these particles, the results appear to indicate considerable sensitivity to not only the source and type of particle, but also variations from within the same lot of material in the case of iron metal powders. In fact, as compared to chromium dioxide particles, the interaction of iron and similar particles with various binders is significantly less. For example, polyurethanes, which are quite suitable when used with chromium dioxide particles, are not acceptable when used as the binder for iron and iron oxide particles. EP-A-463601 discloses a magnetic recording medium having a magnetic recording layer comprising a glassy polymer, a rubbery polymer and an inorganic filler. However the composition does not achieve a reinforced rubber morphology. The present invention makes it possible to overcome problems associated with prior art formulations, and especially with respect to toughness, cohesive integrity and adhesion between the filler and polymer component. According to the present invention we provide a flexible magnetic recording media which comprises a flexible substrate coated with a composition comprising: an inorganic filler having adsorption sites selected from the group consisting of unhydrated oxide sites, hydrated oxide sites and mixture thereof; and a binder component wherein said binder component comprises a glassy polymer having a glass transition temperature of at least 60°C, a molecular weight (Mw) of less than 60,000, a Mn greater than 5000 and having polar groups capable of interacting with adsorption sites on the surface of said filler; and a rubbery polymer having polar groups and being compatible with said glassy polymer wherein the relative amounts of B)1) and B)2) are 50% to 80% by weight of B)1) and correspondingly 20% to 50% by weight of B)2) based upon the total of B)1) and B)2) in the composition; and wherein said glassy polymer and said rubbery polymer are chlorine free; and wherein the relative amounts of A) and B) are 35% to 60% of A) and correspondingly 65% to 40% by volume of B) based upon the total of A) and B) in the composition; and wherein the inorganic filler includes ferromagnetic pigments; and being obtainable by forming a premix of the inorganic filler, the glassy polymer and an organic diluent, which contains a volatile [i.e. having a boiling point less than 90C] polar organic solvent wherein said premix contains 2 to 20% by weight of said glassy polymer, and 20 to 50% by weight of said filler; and admixing said premix and the rubbery polymer; and providing in the composition a high boiling [i.e. having a boiling point greater than 90C] secondary diluent in an amount to provide a blend of diluent of 70 to 90% by weight of said volatile polar organic solvent and correspondingly 10 to 30% by weight of said secondary diluent; and wherein said high boiling secondary diluent is a non solvent for said glassy polymer; and thereby causing said glassy polymer to adsorb onto said inorganic filler and interact with adsorption sites on said filler to thereby encapsulate said filler, and to intertwine said rubbery polymer within the matrix of said glassy polymer. The binder component employed pursuant to the present invention includes a glassy polymer and a rubbery polymer. The glassy polymer has a glass transition temperature of at least 60°C. The glassy polymer also has polar groups capable of interacting with adsorption sites on the surface of the inorganic filler. The rubbery polymer employed has polar groups and is compatible with the glassy polymer. The relative amounts of the glassy polymer and rubber polymer are about 50% to about 80% by weight of the glassy polymer and correspondingly, about 20% to about 50% of the rubbery polymer based upon the total of the glassy polymer and rubber polymer. The relative amounts of the inorganic filler and binder component are about 35% to about 60% by volume, and preferably, about 40% to about 50% by volume of filler and correspondingly, about 65% to about 40% by volume, and preferably, about 50% to about 40% by volume of the binder component. The glassy polymer is in contact with and encapsulates the inorganic filler. Figure 1 illustrates the Tensile Storage Modulus v. Temperature for composition of the present invention, as compared to a composition outside the scope of the invention. Figure 2 illustrates the Tensile Loss Modulus v. Temperature for composition of the present invention, as compared to a composition outside the scope of the invention. Figure 3 illustrates Tensile Storage Modulus v. Temperature for compositions of the present invention, using different solvents. Figure 4 illustrates Tensile Loss Modulus v. Temperature for compositions of the present invention using different solvents. Figure 5 is a schematic illustration of the compositions of the present invention. The compositions of the present invention comprise an inorganic filler and a binder component. The relative amounts of the inorganic filler and binder component are about 35 to about 60% by volume, and preferably, about 40% to about 50% by volume of inorganic filler, and correspondingly about 65 to about 40% by volume, and preferably, about 50% to about 40% by volume of the binder component. These amounts are based upon the total volume of the filler and binder component in the composition. Typical fillers employed when the compositions are to be used for fabricating magnetic recording media are ferromagnetic particles, including iron metal, iron oxide, cobalt-modified iron oxide, and magnetic ferrites, such as barium ferrite, and mixtures thereof. In addition, the filler can include minor amounts, such as up to about 10% by weight, and more typically, when present, about 2 to about 5% of non magnetic particles, such as alumina, zirconia, silica and carbon. The fillers are preferably high surface area particles, such as those having a surface area of at least about 40 m 2 /gm and preferably about 40 m 2 /gm to about 50 m 2 /gm. Certain of the fillers, such as carbon and silica, can have extremely large surface areas, such as up to about 1000 m 2 /gm. The inorganic fillers employed pursuant to the present invention contain adsorption sites, typically unhydrated oxide sites and/or hydrated oxide sites, with which the glassy polymer interacts. The binder component employed includes a glassy polymer and a rubbery polymer. The relative amounts of the glassy polymer and rubbery polymer are 50% to 80%, and preferably about 50% about 60% by weight of the glassy polymer, and correspondingly, to 20% to 50% by weight, and preferably about 40% to about 50% by weight of the rubbery polymer. These amounts are based upon the weight of glassy polymer and rubbery polymer in the binder component. The glassy polymer employed pursuant to the present invention must have a glass transition temperature of at least 60°C, and preferably at least about 80°C, and most preferably about 80°C to about 100°C. The glass transition temperature referred to herein, as would be understood by those skilled in the art, refers to the primary or predominant glass transition temperature. In addition, the glassy polymer must contain polar groups that are capable of interacting with adsorption sites on the filler. Typically, the glassy polymers contain at least about 10 mole % of polar groups in the polymer backbone. Typically, the glassy polymers contain no more than about 60 mole % polar groups in the polymer backbone. Moreover, the glassy polymers can also include pendant or side chain functionality of less than about 2 mole %. Examples of suitable polar groups are ester groups, nitrile groups, amino groups, carboxylate groups (e.g., COOH), sulfonate groups and phosphate groups. The sulfonate and phosphate groups include both the acid, as well as the alkali metal and C 1-4 hydrocarbon derivatives thereof. Examples of suitable glassy polymers are polymers of ethylenically unsaturated monomers or acetylenically unsaturated monomers, such as polymers from vinylacetate; vinylchloride; acrylic acid; methacrylic acid; acrylates including alkyl acrylates, such as ethyl acrylate, methyl acrylate, and butyl acrylate; glycidyl acrylate; and hydroxyalkyl acrylates, such as hydroxyethyl acrylate and hydroxypropyl acrylate; methacrylates including alkyl methacrylates, such as methyl methacrylate; ethyl methacylate and butyl methacrylate; glycidyl methacrylate; and hydroxyalkyl methacrylates such as hydroxyethyl methacrylate, and hydroxymethyl methacrylate; allyl alcohol, olefins such as ethylene, propylene and butylene; styrene; acrylonitrile and copolymers thereof. Also suitable as the glassy polymer are epoxy acrylate or methacrylate reaction products (e.g., reaction products of monoethylenically unsaturated carboxylic acid and bisphenol A-diglycidyl ether epoxide; urethane acrylate reaction products; polycarbonates; phenoxy polymers, phenolics such as phenol - formaldehyde polymers; and cellulosic polymers, such as cellulose acetate and cellulose nitrate. In addition, the glassy polymer can be a polyether polyurethane, a polyester polyurethane, or a polycarbonate polyurethane. In such case, the polar groups are predominantly carbonate groups and ester groups. Typical polyurethanes suitable as the glassy polymer are the same as those disclosed hereinbelow for the rubbery polymer. expect that the amount of hard segment is such that the glass transition temperature is at least 60°C, and typically, contains about 50% to about 65% of hard segment. Furthermore, the glassy polymers employed pursuant to the present invention have a molecular weight (Mw) of less than 60,000 and a Mn greater than 5000. Moreover, in those situations when it is desired to ensure against corrosion of, for instance, head gap metals, the polymers employed are preferably free of chlorine or chloride groups that can possibly result in dehydrahalogenatum and subsequent corrosion. Therefore, in such case, it is preferred not to use vinyl chloride polymers. If desired, the glassy polymer can be curable, such as chemically, with an isocyanate, epoxy and/or with radiation (e.g., electron, beam or actinic light). In the event the glassy polymer is curable, the amount of crosslinking sites should be no more than 10% mole, preferably no more than 5% mole, and most preferably, up to about 2% mole of the glassy polymer. It has been found that larger amounts of crosslinking sites increase stress on the filler to an undesired extent and thereby tending to separate from the filler surface. The rubbery polymer employed pursuant to the present invention must also contain polar groups and be compatible with the glassy polymer. Typical polar groups on the rubber polymer are carbamate groups, ester groups, hydroxyl groups and carboxylate groups, and preferably carbamate. The rubbery polymer can be a polyether urethane, a polyester urethane, or polycarbonate urethane, and preferably, has a molecular weight (Mw) of less than about 50,000 and a Mn of greater than about 5000. The rubbery polymer typically has about 2 to about 20 mole % polar groups in the polymer backbone chain. Also, the rubbery polymer can have pendant or side chain functionality of less than 2 mole %. Typically, the rubbery polymer has a glass transition temperature of about 50°C or less, more typically, about -40°C to about 50°C, and preferably, about 0°C to about 50°C. In addition, the rubbery polymer can include crosslinking sites in an amount of up to about 10 mole %; preferably up to about 5 mole % and, and more preferably, up to about 2 mole %. The polyurethanes are hydrolytically stable and contain a hard segment a id soft segment, as described in U.S. Patents 4,525,424 and 4,568,611, disclosures of which are incorporated herein by reference. However, the polyurethane contains about 20 to 40% of the hard segment arid the molecular weight of the soft segment is about 800 to about 4000. Typical thermoplastic polyurethanes are reaction products of: (A) A hydroxy-terminated polyether, hydroxy terminated polycarbonate, or hydroxyl-terminated polyester. The hydroxy-terminated polyethers, polyesters and polycarbonates exhibit a hydroxyl number of about 50 to about 260; (B) a diol chain extender (typically 1,4-butanediol; 1,3-butanediol; 1.5-pentanediol; 1,6-hexanediol; and 2,5-hexanediol, and mixtures of 1,4-butanediol arid 1,6-hexanediol), in order that the hydroxyl number of the polyester and the chain extender together is about 130 to about 350; and (C) an aromatic diisocynate (4,4'-diphenylmethane diisocyanate (MDI) is typical. In addition, the constituents (A), (B) and (C) are used in a proportion to yield a polyurethane of hard segment content in the range of about 20 to 40 percent by weight. Exemplary hydroxy-terminated polyesters are reaction products of a hydroxyticallly stable difunctional alcohol and a dicarboxylic acid. Typical difunctional alcohols are 1,4-cyclohexane-dimethanol, ε-caprolactane,and 2,3 dimethyl 1,3 propane diol. Typical dicarboxylic acids are adipic, azelaic, 1,12-dodecanediodic acid, hexanedioic acid, nonanedioic acid and octanedioic acid. Examples of some specific suitable glassy polymers and rubbery polymers, along with a discussion of certain properties thereof, are given in Table 1 herein below. Moreover, the selection of particular glassy polymers and rubbery polymers can be carried out by those skilled in the art, once aware of the above disclosure of polymer parameters, and requirements without undue experimentation. Therefore, such need not be described herein in any greater detail. In addition, as known in the art, the composition can include auxiliary components such as lubricants, dispersants and other minor additives. It is critical to the success of the present invention that the glassy polymer interact with adsorption sites on the surface of the inorganic filler, so that such is in contact with and encapsulates the inorganic filler. In order to accomplish this, the glassy polymer defined above, the inorganic filler and an organic diluent that contains a volatile polar organic solvent is provided in a premix. The volatile polar solvent typically has a boiling point of less than 90°C, and is capable of dispersing the inorganic filler and glassy polymer. Typical volatile polar solvents include acetone, methyl ethyl ketone, and preferably tetrahydrofuran. If desired, the organic diluent can also contain a secondary higher boiling diluent that is a non solvent for the glassy polymer. However, the organic diluent should not contain more than 20% by weight of the higher boiling secondary diluent. Such higher boiling diluent has a boiling point greater than 90°C, and includes higher boiling ketones, such as methylisobutyl ketone, acetates such as amyl acetate and ethyl acetate, hydrocarbons, including octane, isooctane and decane, and aromatic solvents such as xylene and toluene. The secondary diluent is a non solvent for the glassy polymer, but can be a partial solvent for the rubbery polymer. The lower volatile polar solvent is a better solvent for the glassy polymer than is the secondary diluent. If curable sites are present on the glassy polymer and/or rubbery polymer, such sites should preferentially phase separate onto the secondary diluent, such that subsequent cross-linking occurs between the glassy region and rubbery region. The premix typically contains about 20 to about 50% by weight of the inorganic filler, and about 2 to about 20% by weight of the glassy polymer in the premix, with the remainder being the organic diluent, and if desired, the auxiliary ingredients, if employed. The premix is agitated so as not to cause destruction of the inorganic filler therein. The rubbery polymer and premix are combined. In addition, the secondary diluent is included to provide a diluent blend that contains about 70 to about 90% by weight of the volatile polar solvent, and correspondingly about 10 to about 30% by weight of the secondary diluent. Upon coating the composition onto a substrate, and upon evaporation of the more volatile solvent, the glassy polymer is caused to adsorb onto the inorganic filler and interact with adsorption sites on the inorganic filler to thereby encapsulate the filler. At the same time, since the rubbery polymer is soluble or at least swellable in the higher boiling secondary diluent, such can absorb stresses of the drying without disruption of the particle/glassy resin interface. By controlling the phase separation and composite morphology, a tough, cohesive, high modulus coating is provided. The glassy resin is fabricated into a cohesive, flexible coating, by linking the filler encapsulated hard phases with a rubbery phased component to form a reinforced-rubber type mechanical response. The rubbery polymer becomes intertwined with the glassy resin. Also, depending upon the chemical nature of the glassy resin and rubbery polymer, there may be some crosslinking between them. Figure 5 schematically illustrates the configuration of the filled compensation pursuant to the present invention. A typical substrate when providing a flexible magnetic recording media is a flexible substrate, such as polyethylene terephthalate to form magnetic recording tape. The following non-limiting examples are presented to further illustrate the present invention. In particular, Fig. 1 and Fig. 2 show a comparison between two compositions (Example 1 comparative and Example 2) having the same ingredients and amounts of glassy polymers and rubbery polymers, but prepared in different ways. The composition of Example 2 has been prepared according to the procedure of forming a premix of the glassy polymer and inorganic filler described in the preferred embodiment of the present invention. Therefore the two examples have different final composition configurations. The glassy polymer employed in both examples 1 and 2 is VCAR VROH and the rubbery polymer is Estane 5706. The results of Tensile Storage Modulus v. Temperature are shown in Figures 1 and 2, respectively, and clearly illustrate the significantly improved results achieved by the present invention. In particular, the glass transition temperatures achieved by the present invention are significantly higher than those obtained outside the scope of this invention. Also, the mechanical response achieved by the present invention is more representative of a rubber reinforced polymer composition. Example 3 relates to compositions according to the present invention employing either methyl isobutyl ketone or toluene with the results of Tensile Storage Modulus v. Temperature; and Tensile Loss Modulus v. Temperature shown in Figures 3 and 4, respectively. The glassy resin employed was VCAR VROH and the rubber polymer employed was Estane 5706. Another possible composition according to the present invention is obtained by using alumina as filler, VAGH as glassy resin and Estane 5706 as rubber polymer.

Claims: A flexible magnetic recording media which comprises a flexible substrate coated with a composition comprising: a) an inorganic filler having adsorption sites selected from the group consisting of unhydrated oxide sites, hydrated oxide sites and mixture thereof; and b) a binder component wherein said binder component comprises 1) a glassy polymer having a glass transition temperature of at least 60°C, a molecular weight (Mw) of less than 60,000, a Mn greater than 5000 and having polar groups capable of interacting with adsorption sites on the surface of said filler; and 2) a rubbery polymer having polar groups and being compatible with said glassy polymer wherein the relative amounts of B)1) and B)2) are 50% to 80% by weight of B)1) and correspondingly 20% to 50% by weight of B)2) based upon the total of B)1) and B)2) in the composition; and wherein said glassy polymer and said rubbery polymer are chlorine free; and wherein the relative amounts of A) and B) are 35% to 60% of A) and correspondingly 65% to 40% by volume of B) based upon the total of A) and B) in the composition; and wherein the inorganic filler includes ferromagnetic pigments; and being obtainable by forming a premix of the inorganic filler, the glassy polymer and an organic diluent, which contains a volatile [i.e. having a boiling point less than 90C] polar organic solvent wherein said premix contains 2 to 20% by weight of said glassy polymer, and 20 to 50% by weight of said filler; and admixing said premix and the rubbery polymer; and providing in the composition a high boiling [i.e. having a boiling point greater than 90C] secondary diluent in an amount to provide a blend of diluent of 70 to 90% by weight of said volatile polar organic solvent and correspondingly 10 to 30% by weight of said secondary diluent; and wherein said high boiling secondary diluent is a non solvent for said glassy polymer; and thereby causing said glassy polymer to adsorb onto said inorganic filler and interact with adsorption sites on said filler to thereby encapsulate said filler, and to intertwine said rubbery polymer within the matrix of said glassy polymer. The flexible magnetic recording media of claim 1 wherein said inorganic filler has a surface area of at least 40 m 2 /gm. The flexible magnetic recording media of any preceding claim, wherein said filler contains iron, iron oxide, magnetic ferrite, cobalt-modified iron oxide, or mixtures thereof. The flexible magnetic recording media of any preceding claim wherein the relative amounts of A) and B) are 40% to 50% by volume of A), and correspondingly, 50% to 40% by volume of B), based upon the total volume of A) and B) in the composition. The flexible magnetic recording media of any preceding claim wherein the glass transition temperature of said glassy polymer is at least 80°C. The flexible magnetic recording media of any preceding claim wherein the glass transition temperature of said glassy polymer is 80°C to 100°. The flexible magnetic recording media of any preceding claim wherein said glassy polymer contains at least 10 mole % of said polar groups in the polymer backbone. The flexible magnetic recording media of any preceding claim wherein said glassy polymer contains up to 60% of said polar groups in the polymer backbone. The flexible magnetic recording media of claim 7 wherein said glassy polymer contains pendant side chain functionality of less than 2 mole %. The flexible magnetic recording media of claim 7 wherein said glassy polymer contains up to 10 weight percent of crosslinking sites. The flexible magnetic recording media of any preceding claim wherein said rubbery polymer has 2 to 20 mole % of polar groups in the polymer backbone chain, a molecular weight (Mw) of less than 50,000, and a Mn of greater than 5000. The flexible magnetic recording media of any preceding claim wherein said rubbery polymer is a polyurethane.

Inventors: LIONEL BRADSHAW RICHARD, LIONEL, BRADSHAW RICHARD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1994
Main classification: C08G18
IPC codes: C08K3, C08L101, C08J5, G11B5, C08L75, C09D5, C08L21